When it is desirable for a radio frequency (RF) chip, such as a radio frequency identification (RFID) chip, to use an on-chip antenna, one of the most efficient designs to use is a resonant small loop antenna. A small loop antenna is one with a total radiator length (or circumference for a single turn loop antenna) that is less than one quarter of a wavelength. Very often these on-chip loop antenna radiating elements are formed with the largest area single turn loop of a low resistance wire that is broken at one point and, thus, the single turn loop is seen as a one turn inductor. Such loop antennas typically have a tuning capacitor connected across the break and having a capacitance chosen to resonate the antenna at the desired use frequency.
An on-chip loop antenna would work very well if it were not for the usual requirement of having a metallic chip seal ring, or guard-ring, around the perimeter of the integrated circuit. The main purpose of the seal ring is to prevent ionic or other contamination from entering the integrated circuit aboard the chip, since ionic or other contamination can cause a degradation of the operating characteristics of components, such as transistors, within the integrated circuit chip. The presence of the seal ring can significantly reduce the effectiveness of the loop antenna by virtue of the antenna magnetically inducing currents into the seal ring that are opposite to those in the antenna, thereby canceling a sometimes large portion of the magnetic field emanating from the antenna. Also, because the plates of the capacitor on the antenna carry the largest alternating current (AC) voltage as compared with any other point on the antenna, when there is capacitive coupling from the loop antenna near the capacitor to the nearby chip seal ring, the capacitor voltage is shorted out. One solution may be to cut a small gap in the chip seal ring in the region that is adjacent to the antenna capacitor. However, this is typically not permitted in chip manufacturing because of reliability concerns from ionic or other contamination, as described above.